Description

The malloc() function returns a pointer to a block of at least
size bytes suitably aligned for any use.

The argument to free() is a pointer to a block previously allocated
by malloc(). After free() is performed, this space is made available for
further allocation, and its contents have been destroyed. See mallopt() below for a
way to change this behavior. If ptr is a null pointer, no
action occurs.

Undefined results occur if the space assigned by malloc() is overrun or
if some random number is handed to free().

The free() function does not set errno.

The memalign() function allocates size bytes on a specified alignment boundary and
returns a pointer to the allocated block. The value of the returned
address is guaranteed to be an even multiple of alignment. The value
of alignment must be a power of two and must be greater
than or equal to the size of a word.

The realloc() function changes the size of the block pointed to by
ptr to size bytes and returns a pointer to the (possibly moved)
block. The contents will be unchanged up to the lesser of the
new and old sizes. If the new size of the block requires movement
of the block, the space for the previous instantiation of the block
is freed. If the new size is larger, the contents of the
newly allocated portion of the block are unspecified. If ptr is NULL,
realloc() behaves like malloc() for the specified size. If size is 0 and
ptr is not a null pointer, the space pointed to is freed.

The valloc() function has the same effect as malloc(), except that the
allocated memory will be aligned to a multiple of the value returned
by sysconf(_SC_PAGESIZE).

The calloc() function allocates space for an array of nelem elements of
size elsize. The space is initialized to zeros.

The mallopt() function provides for control over the allocation algorithm. The available
values for cmd are:

M_MXFAST

Set maxfast to value. The algorithm allocates all blocks below the size of maxfast in large groups and then doles them out very quickly. The default value for maxfast is 24.

M_NLBLKS

Set numlblks to value. The above mentioned ``large groups'' each contain numlblks blocks. numlblks must be greater than 0. The default value for numlblks is 100.

M_GRAIN

Set grain to value. The sizes of all blocks smaller than maxfast are considered to be rounded up to the nearest multiple of grain. grain must be greater than 0. The default value of grain is the smallest number of bytes that will allow alignment of any data type. Value will be rounded up to a multiple of the default when grain is set.

M_KEEP

Preserve data in a freed block until the next malloc(), realloc(), or calloc(). This option is provided only for compatibility with the old version of malloc(), and it is not recommended.

These values are defined in the <malloc.h> header.

The mallopt() function can be called repeatedly, but cannot be called after
the first small block is allocated.

The mallinfo() function provides instrumentation describing space usage. It returns the mallinfo
structure with the following members:

unsigned long arena; /* total space in arena */
unsigned long ordblks; /* number of ordinary blocks */
unsigned long smblks; /* number of small blocks */
unsigned long hblkhd; /* space in holding block headers */
unsigned long hblks; /* number of holding blocks */
unsigned long usmblks; /* space in small blocks in use */
unsigned long fsmblks; /* space in free small blocks */
unsigned long uordblks; /* space in ordinary blocks in use */
unsigned long fordblks; /* space in free ordinary blocks */
unsigned long keepcost; /* space penalty if keep option */
/* is used */

The mallinfo structure is defined in the <malloc.h> header.

Each of the allocation routines returns a pointer to space suitably aligned
(after possible pointer coercion) for storage of any type of object.

Return Values

The malloc(), memalign(), realloc(), valloc(), and calloc() functions return a null pointer
if there is not enough available memory. When realloc() returns NULL, the
block pointed to by ptr is left intact. If size, nelem, or
elsize is 0, either a null pointer or a unique pointer that
can be passed to free() is returned. If mallopt() is called after
any allocation or if cmd or value are invalid, a non-zero value
is returned. Otherwise, it returns 0.

Errors

If malloc(), calloc(), or realloc() returns unsuccessfully, errno is set to
indicate the error:

ENOMEM

size bytes of memory exceeds the physical limits of your system, and cannot be allocated.

EAGAIN

There is not enough memory available at this point in time to allocate size bytes of memory; but the application could try again later.

Usage

Unlike malloc(3C), this package does not preserve the contents of a block
when it is freed, unless the M_KEEP option of mallopt() is used.

Function prototypes for malloc(), realloc(), calloc(), and free() are also defined in
the <malloc.h> header for compatibility with old applications. New applications should
include <stdlib.h> to access the prototypes for these functions.

Comparative features of the various allocation libraries can be found in the
umem_alloc(3MALLOC) manual page.